1. Field of the Invention
The present invention relates to a matrix array substrate including a non-linear resistive device, more specifically to an MIM (Metal-Insulator-Metal) element, serving as a switching device, and a method of manufacturing the same.
2. Description of the Related Art
Recently, liquid crystal display devices have used in relatively simple devices such as watches, calculators, various types of measuring instruments, and even in devices handling a great amount of data such as office automation terminal devices and television sets. In the field of the liquid crystal devices, improvement of the quality of images has been of the greatest interest. Presently, there are many types of switching arrays proposed for those devices. Of all the types, the one with a bi-terminal non-linear resistive device is very popular because of its simple structure and easy assembling procedure, and more specifically, an MIM type element array can be provided for its present commercial success.
FIG. 1 is a cross section of a pixel of an array substrate having an MIM element, and the MIM element is produced as follows:
A Ta film 2 is formed on a glass substrate by a thin film formation method such as sputtering or vacuum deposition, or the like. Then, the Ta film 2 is patterned by a photo-lithography, thereby forming a wiring and one of the two electrodes of an MIM element. After that, the surface of the Ta film 2 is oxidized in an aqueous solution of citric acid by the anodic oxidation method, forming an oxide film 3. Further, a Cr film 4, which serves as the other electrode, is formed by the thin film formation method, thereby completing an MIM element. Lastly, a transparent picture element electrode 5 for image display is formed such that it is connected to the MIM element.
A basic technique of manufacturing such an array substrate is disclosed in Published Examined Japanese Patent Application (PEJPA) No. 1-35352, and a remodeled version of this technique is set forth in Published Unexamined Japanese Patent Application (PUJPA) No. 58-35352.
In conventional MIM elements, as set forth in PEJPA No. 1-35352, the same metal is used for one of the electrodes and the wiring. Consequently, it is not always assured that a material having a small electric resistance is used. Although the .beta.-type tantalum widely used is an excellent material for an MIM element, it has a high resistance. If the resistance of a wiring electrode is high, the waveform of a driving pulse applied from an external drive circuit is distorted. As a result, deterioration and decline of contrast occurs due to lack of writing. Especially, in the case where a display screen or display capacity is large, the width and length of the wiring become(narrow and long, rendering the above problem worse. Thus, in order to produce a device of a high performance, which can display a uniform image, it is essential in a practical sense that the resistance of the wiring electrode is reduced to a certain level.
However, if different metals are used for the one of the electrodes of the MIM element, and the wiring electrode, some additional steps are required. Such addition of steps complicate the manufacturing procedure. Therefore, the reduction of the resistance must be achieved by a slightest possible modification to the conventional materials and procedure.